The term inert pair effect is often used in relation to the increasing stability of oxidation states that are two less than the group valency for the heavier elements of groups 13, 14, 15 and 16. The term "inert pair" was first proposed by Nevil Sidgwick in 1927. The name suggests that the outermost s electrons are more tightly bound to the nucleus in these atoms, and therefore more difficult to ionize or share.
For example, the p-block elements of the 4th, 5th and 6th period come after d-block elements, but the electrons present in the intervening d- (and f-) orbitals do not effectively shield the s-electrons of the valence shell. As a result, the inert pair of ns electrons remains more tightly held by the nucleus and hence participates less in bond formation .
Consider as an example thallium (Tl) in group 13. The +1 oxidation state of Tl is the most stable, while TlIII compounds are comparatively rare. The stability of the +1 oxidation state increases in the following sequence:
The same trend in stability is noted in groups 14, 15 and 16. The heaviest members of each group, i.e. lead, bismuth and polonium are comparatively stable in oxidation states +2, +3, and +4 respectively.
The lower oxidation state in each of the elements in question has 2 valence electrons in s - orbitals. On the face of it, a simple explanation could be that the valence electrons in an s orbital are more tightly bound and are of lower energy than electrons in p orbitals and therefore less likely to be involved in bonding. Unfortunately this explanation does not stand up. If the total ionization potentials (IP) (see below) of the 2 electrons in s orbitals (the 2nd + 3rd ionization potentials), are examined it can be seen that they increase in the sequence:
|(2nd + 3rd)||6086.8||4561.5||4942.3||4524.6||4849|
The high ionization potential (IP) (2nd + 3rd) of gallium is explained by d-block contraction, and the higher IP (2nd + 3rd) of thallium relative to indium, has been explained by relativistic effects.
An important consideration is that compounds in the lower oxidation state are ionic, whereas the compounds in the higher oxidation state tend to be covalent. Therefore, covalency effects must also be taken into account. In fact an alternative explanation of the inert pair effect by Drago in 1958 attributed the effect to low M-X bond enthalpies for the heavy p-block elements and the fact that it requires less energy to oxidize an element to a low oxidation state than to a higher oxidation state. This energy has to be supplied by ionic or covalent bonds, so if bonding to a particular element is weak, the high oxidation state may be inaccessible. Further work involving relativistic effects confirms this. In view of this it has been suggested that the term inert pair effect should be viewed as a description rather than as an explanation.
The chemical inertness of the s electrons in the lower oxidation state is not always married to steric inertness (where steric inertness means that the presence of the s electron lone pair has little or no influence on the geometry of the molecule or crystal). A simple example of steric activity is that of SnCl2 which is bent in accordance with VSEPR. Some examples where the lone pair appears to be inactive are bismuth(III) iodide, BiI3, and the anion. In both of these the central Bi atom is octahedrally coordinated with little or no distortion, in contravention to VSEPR theory. The steric activity of the lone pair has long been assumed to be due to the orbital having some p character, i.e. the orbital is not spherically symmetric. More recent theoretical work shows that this is not always necessarily the case. For example, the litharge structure of PbO contrasts to the more symmetric and simpler rock salt structure of PbS and this has been explained in terms of PbII − anion interactions in PbO leading to an asymmetry in electron density. Similar interactions do not occur in PbS. Another example are some thallium(I) salts where the asymmetry has been ascribed to s electrons on Tl interacting with antibonding orbitals.
Part of the rationale for describing this as an effect was the fact that, at the time when it was proposed, there were no known compounds of Group 13 elements with the intermediate, +2, oxidation state. This is no longer true since the discovery of certain complexes of Ga(II) and In(II), such as halides of the form [M2X6]2-. These complex ions are stabilized by the formation of a covalent M–M bond. It follows that the instability of simple complexes of ions such as Ga2+ is due to kinetic factors, namely that the Ga2+, having an unpaired electron, behaves as a free radical and is rapidly destroyed by reaction with another free radical. If Drago's explanation as described above were correct, these compounds would be thermodynamically stable by virtue of the formation of a covalent bond between the gallium ions.